Fingers leave tell-tale clues about you on your phone

Chemical traces on a cell phone allow researchers to construct lifestyle sketches of its owner.

Amina Bouslimani and Neha Garg

Every time your fingers touch your cell phone, they leave behind trace amounts of chemicals. And each chemical offers clues to you and your activities. By analyzing them, forensic scientists might be able to piece together a story about your recent life, a new study finds. One day, police might use such data to help track down a phone’s owner. Or they might figure out what a person had recently been up to.

A molecule is a group of atoms. It represents the smallest amount of some chemical. Your skin is covered in molecules picked up by everything you have touched. Those molecules might include traces of the chocolate bar you had snacked on. Or there might be small amounts of shampoo, cosmetics — even some medicine you took. And with each new thing your skin contacts, you leave behind some small share of what it had touched earlier.

Researchers at the University of California, San Diego (UCSD) recently analyzed such chemical leftovers on the phones of 39 volunteers. These residues helped the scientists analyze each phone user’s behavior.

Amina Bouslimani led the study. As a biochemist, she studies the chemical processes that take place inside organisms. Her team’s new tests often could discover if a phone’s owner liked spicy food, drank coffee or used deodorant. The tests could point to places someone had visited recently. They could even point to whether he or she was ill.

Bouslimani’s team published its findings online November 14 in the Proceedings of the National Academy of Sciences.

"Like your skin, your phone also reflects who you are and what you do," she says. That's because the average person spends about five hours a day handling their cell phone, she notes.

Jack Gilbert is a microbial ecologist at the University of Chicago in Illinois. He studies how microbes relate to each other and their surroundings. The UCSD research is exciting, he says. That’s because it allows scientists to reconstruct details of a phone owner’s lifestyle.

Consider someone who has just lunched on a peanut butter and jelly sandwich. "A small amount of the jelly, the bread, the butter and even the peanut would be left on your hands,” he says. (Some people’s PB and J sandwiches also contain butter.) That would be true “even after you have washed them," he notes. In fact, washing would add traces of soap.

Residues from each recent activity would increase the complexity of the mix of molecules left behind.

What the scientists did

To probe those residues, the UCSD team wiped the surface of each volunteer's phone with a cotton swab. The scientists also swabbed each person’s right hand. Then the researchers compared the chemicals found on each.

If the chemicals matched, it would suggest that those molecules from the skin had been transferred to a user’s phone, says co-author Pieter Dorrestein. He’s a UCSD pharmaceutical chemist. (That means he studies the composition, structure and properties of different types of medicines.)

The scientists identified as many of the molecules as they could. They then compared these to a database of chemicals. Dorrestein had helped set up that database a few years earlier. It contains the profiles of various compounds, including spices, caffeine and medicines.

Traces of anywhere from hundreds to thousands of different molecules turned up on each phone. The molecules reflected what had been in the body, such as medications and food. They also reflected what each person had handled before touching the phone, such as soap or makeup. Indeed, the majority of molecules came from beauty products, medicines and food.

From all of this, Bouslimani says, "We could tell if a person is likely female, uses high-end cosmetics, dyes her hair, drinks coffee, prefers beer over wine or likes spicy food." These residues might also show whether someone wears sunscreen or bug spray. If so, that could point to someone who spends a lot of time outdoors.

Police already use molecular analyses to look for traces of explosives or illegal drugs. To date, Dorrestein says, he's never heard of police using phone residues to narrow down behavioral clues to search for a suspect. But detectives might one day use such data to track down someone who left a phone behind at a crime scene.

Power Words

atom The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and neutrally charged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

behavior The way a person or other organism acts towards others, or conducts itself.

caffeine A stimulant, which activates the nervous system and heart. The leaves, seeds and fruits of many plants contain caffeine. In coffee plants and tea bushes, caffeine acts as a natural pesticide. It will kill or harm insects that attempt to dine on the plant. Caffeine is also toxic to some types of plants, bacteria — even frogs and dogs.

chemical A substance formed from two or more atoms that unite (become bonded together) in a fixed proportion and structure. For example, water is a chemical made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H2O. Chemical can also be an adjective that describes properties of materials that are the result of various reactions between different compounds.

chemistry The field of science that deals with the composition, structure and properties of substances and how they interact with one another. Chemists use this knowledge to study unfamiliar substances, to reproduce large quantities of useful substances or to design and create new and useful substances. (about compounds) The term is used to refer to the recipe of a compound, the way it’s produced or some of its properties. People who work in this field are known as chemists.

compound (often used as a synonym for chemical) A compound is a substance formed from two or more chemical elements united in fixed proportions. For example, water is a compound made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H2O.

data Facts and/or statistics collected together for analysis but not necessarily organized in a way that gives them meaning. For digital information (the type stored by computers), those data typically are numbers stored in a binary code, portrayed as strings of zeros and ones.

database An organized collection of information.

mean One of several measures of the “average size” of a data set. Most commonly used is the arithmetic mean, obtained by adding the data and dividing by the number of data points.

microbe Short for microorganism. A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell.

molecule An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

online A term that refers to things that can be found or done on the Internet.

organism Any living thing, from elephants and plants to bacteria and other types of single-celled life.

pharmaceuticals Medicines, especially prescription drugs.

Proceedings of the National Academy of Sciences A prestigious journal publishing original scientific research, begun in 1914. The journal's content spans the biological, physical, and social sciences. Each of the more than 3,000 papers published each year, now, not only is peer reviewed but also approved by a member of the U.S. National Academy of Sciences.

residue A remnant or material that is left behind after something has been removed. For instance, residues of paint may remain behind after someone attempts to sand a piece of wood; or sticky residues of adhesive tape may remain on the skin after a bandage is removed; or residues of chemicals may remain in the blood after exposure to a pollutant.

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